Mesoporous Pt@Pt-skin Pt3Ni core-shell framework nanowire electrocatalyst for efficient oxygen reduction

The design of Pt-based nanoarchitectures with controllable compositions and morphologies is necessary to enhance their electrocatalytic activity. Herein, we report a rational design and synthesis of anisotropic mesoporous Pt@Pt-skin Pt3Ni core-shell framework nanowires for high-efficient electrocatalysis. The catalyst has a uniform core-shell structure with an ultrathin atomic-jagged Pt nanowire core and a mesoporous Pt-skin Pt3Ni framework shell, possessing high electrocatalytic activity, stability and Pt utilisation efficiency. For the oxygen reduction reaction, the anisotropic mesoporous Pt@Pt-skin Pt3Ni core-shell framework nanowires demonstrated exceptional mass and specific activities of 6.69 A/mgpt and 8.42 mA/cm2 (at 0.9 V versus reversible hydrogen electrode), and the catalyst exhibited high stability with negligible activity decay after 50,000 cycles. The mesoporous Pt@Pt-skin Pt3Ni core-shell framework nanowire configuration combines the advantages of three-dimensional open mesopore molecular accessibility and compressive Pt-skin surface strains, which results in more catalytically active sites and weakened chemisorption of oxygenated species, thus boosting its catalytic activity and stability towards electrocatalysis.

3. The authors failed to describe how the d-band center was obtained from the data presented in Figure  2f. The interpretation of the d-band center is also problematic because the energy of d-band center is referenced to the Fermi level, i.e., a negative number. If the 0 binding energy in Figure 2f is the Fermi level, then the Pt nanowires actually have the lowest d-band center energy of -2.84 eV while the Pt@Ptskin Pt3Ni CSFWs have the highest d-band center energy. The "valence band XPS" in the figure caption should be corrected to "valence band UPS"? The authors can refer to the following reference for more details. 4. Line 158. The author claims the HRTEM images in Fig. 3h confirmed a well-defined Pt-skin formed on top of the ultrathin Pt3Ni curves framework shells. However, This image is problematic. The inset is a dark field image while Fig. 3h apparently, is a bright field image. In a bright field image, Z-contrast does not exist. So one cannot conclude that the red-doted region is Pt-rich. Furthermore, it is very hard to tell where exactly Fig h is located in the inset. They don't seem to be coming from the same area. So the repetitive claim of Pt skin@ Pt3-Ni has little supporting evidence. It is suggested that the author tune down their emphasis on the skin structure or provide stronger evidence of the claimed Pt-skin, including those mentioned in the discussion of XRD. 5. Please execute reproducibility studies on the RDE testing. Error bars should be provided in the bar chart (Fig 4d) 6. The performance of Pt/C and Pt nanowires risk underestimation since the poor maintenance of limiting current typically reflects a poorly deposited catalyst film on the RDE electrode. This causes a mass transfer problem and resulted in the early experiencing of diffusion resistance (mixed diffusion/kinteic region) in the catalyst layer. The authors should re-do the measurements for Pt/C and Pt nanowires to provide a reasonable control group. 7. The author should explain how the mass loading of Pt on the RDE is determined since the errors would be amplified twice. A higher loading would give a much better kinetic current in the ORR measurement. Therefore, if the loading was underestimated, the mass activity would be overestimated significantly. The relation between catalyst loading and measured mass activity is typically non-linear. To obtain a fair comparison, the electrode loading should be the same (difference within 10%).
In addition, based on the provided CV curves in Figure 4a, the Pt@Pt-skin Pt3Ni CSFWs showed a double layer much larger than other samples. Although it is possible that the porous framework may exhibit more surfaces compared with other types of Pt nanostructures, carbon black should still dominate the double layer region due to the rather low loading of Pt (20wt%). The reviewer highly suspects that there are substantial errors in the determination of the electrode loading. It is recommended, if accessible, that the authors utilized instruments like XRF to directly measure the loading of Pt on the catalystcoated RDE.
8. The Tafel slopes are rather similar for all the nanowire-based catalysts. How come the performances are so different? In the potential range from 0.8 -0.96 V, the more active catalysts seem to start from the mass transfer limited region while the less active ones started from the mixed kinetic/diffusion region. How is it possible that a straight line can be obtained for all four catalysts in this potential region in the Tafel plot? 9. How come there the Pt oxide reduction peak is more pronounced during the CO stripping experiment? CO-stripping results should also be provided for other samples.
10. The accelerated durability cycles protocol is composed of 50,000 CV cycles from 0.6 to 1.1 V vs RHE. The reviewer is quite surprised that there is almost no sign of carbon corrosion based on the CV provided before and after the ADT. Some degree of carbon corrosion was observed for the Pt@Pt3Ni CSFWs/C while severe carbon corrosion occurred on the Pt/C. It would be nice if the authors could explain more on this part. The durability testing results for the Pt nanowires should be provided.
11. What was the process applied to the catalyst after the ADT to recover the recoverable losses? Was it applied to all catalyst samples in the same way?
Reviewer #2 (Remarks to the Author): In their manuscript entitled "Mesoporous Pt@Pt-skin Pt<sub>3</sub>Ni Core-shell Framework Nanowires for High-Efficient Electrocatalysis" Hui Jin et al. report the design and electrocatalytic oxygen reduction activity (ORR) for anisotropic mesoporous Pt@Pt-skin Pt<sub>3</sub>Ni core-shell framework nanowires (CSFWs). While the ORR activity and durability of the reported catalyst are very good and the one-pot synthesis appears to be promising, I cannot recommend this manuscript in its actual form for publication in Nature Communications for the following reasons • Other than the achieved shape of the catalyst, I cannot find new aspects in the research that would justify publication in Nature Communications, the synthesis method (solvothermal) has been reported already for synthesis of shape-controlled NP • The authors must provide a much more detailed description of the various effects of time/temperature/molar ratios of capping/reducing agents onto the shape of the final catalyst, as for example shown by Yong Yang et al. (J. Phys. Chem. C 2007, 111, 26, 9095-9104), combined with the various achieved structures and possibly a volcano plot of the corresponding ORR activities. Without this essential information, it is very difficult to understand the various factors that lead to the shape and activity of the final catalyst • While the reported ORR activity appears impressive compared to 20wt% Pt/C, a comparison to otherhighly active shape controlled -Pt<sub>3</sub>Ni/C catalyst, for example in a tabulated form, is missing, which would allow direct ORR activity comparison with published results. As for example, Pt subnanometer Pt alloy wires with 4.20 A/mg & 5.11 mA/cm<sup>2</sup> at 0.9 V vs RHE and very high durability over 30000 cycles were already reported in 2017 (Science Advances, https://doi.org/10.1126/sciadv.1601705) • Structural data as for example spatial dimensions lack error information Specifically, I would like to add the following questions/comments to the authors: • Lines 77/78: Standard reduction potentials are given for specific conditions (pH) and aqueous solutions of the metals, therefore under the given reaction conditions they vary. Secondly, what exactly triggers the onset of Ni-reduction after the initial Pt-reduction, only the depletion of Pt cations?
• Line 96: To my eye the EDS line profile indicates the presence of Ni in the initially formed Pt wires, while I miss the corresponding EDS mapping images for the wires • Line 124: The observed XRD splitting can also be interpreted as phase separation or possibly some dealloying in the synthesis process • Line 129: This is confusing because the final synthesis product, Pt@Pt-skin Pt<sub>3</sub>Ni CSFWs are obtained after the annealing step, to obtain the Pt-skin structure. If a slight negative shift of 0.4º was observed for the final catalyst, why are the data not presented in figure 2 e?
• Lines 155-160: The Pt/Ni distributions are not very clearly visible from the provided HRTEM and EELS mapping images, the red color for Pt is too dark • Line 164: To prove the role of Ni in the formation of compressive strain of the Pt-skin, the initial Ni content should be varied, and the resulting structures analyzed (c.f., one of my main concerns stated above) • Line 180: The authors should provide a reference for the assumed reaction for the formation of absorbed hydroxyl species • Line 180: The CV figures in figure 4 a are too small to observe any differences in potential onsets • Lines 222-223: Why would the Pt-skin structure protect the electrocatalyst against further Ni-leaching from the inner region of the framework walls? Did the authors try to leach Ni by electrochemical cycling and try to determine a maximum of cycles after which no further Ni leaching was observed?
• Line 379: What do the 20% refer to, Pt or catalyst load?
• Lines 401-403: What was the preference to carry out the LSV in anodic correction? Was any iRcompensation applied?

General Comments
The authors show the synthesis of mesoporous Pt@Pt-skin Pt3Ni core-shell framework nanowires (CSFWs). The structural evolution of the materials from Pt nanowire to Pt@Pt-Ni nanowire intermediate, Pt@Pt-Ni alloy core-shell nanowire, Pt@Pt-skin Pt3Ni CSFWs are thoroughly studied and presented. Materials characterization including electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy was carried out to identify the morphological features, crystal structure, and electronic/valence state of the as-prepared samples. Detailed HAADF-STEM was carried out on the Pt@Pt-skin Pt3Ni CSFWs to investigate the spatial distribution of Pt and Ni elements. Rotating disk electrode (RDE) half-cell study was performed to evaluate the catalytic materials' activity and durability for oxygen reduction reaction (ORR) under acidic conditions. The Pt@Pt-skin Pt3Ni CSFWs showed very impressive mass activity as well as durability. Although the manuscript presents very compelling results with a well-organized presentation, it does not meet the high standards to be published in a prestigious journal like Nature Communications in its present form. Two major issues need to be taken care of. First, the authors repeatedly claim the Pt-skin on Pt3Ni structure but failed to provide direct evidence. In the reviewer's opinion, the HAADF-STEM in Figure  3h is problematic with the arguments provided in the point-to-point comments. Second, although showing very impressive ORR performance and durability, the reproducibility of the RDE testing results is not provided. In addition, the presented data sets are not complete, and certain results for certain samples were missing. Therefore, the reviewer suggests the manuscript be rejected and resubmitted after a major revision addressing the two major problems mentioned above and the point-to-point comments provided below.

Response:
We appreciate the reviewer for the comments. The two major issues raised by the reviewer have been well settled in the revised manuscript. First, extensive efforts have been devoted to well proving the Pt-skin on Pt3Ni structure by HAADF-STEM images, EDS mapping & line-scanning profile and CO stripping results (Please see our detailed responses to Comments 1-4). Second, motivated by the reviewer's kind remind, a complete set of ORR tests (including the reproducibility of the RDE testing, etc.) has been supplemented for a better demonstration of the electrochemical results (Please see our detailed responses to Comments 5-11). Given these efforts, we believe the revised manuscript reserves the high quality and can meet the high standards in Nature Communications.

Response:
We thank the reviewer for raising this comment. We are fully aware that the lattice compression in Pt-skin shell could not be precisely determined by XRD. In the present work, the compressive strain in Pt-skin shell only affects a few atomic layers on the surface of the Pt@Pt-skin Pt3Ni CSFWs sample, resulting in its absence of significant peak shift of the XRD pattern when compared to Pt@Pt3Ni CSFWs (please see Supplementary Figure 16). Therefore, it is really difficult to determine lattice compression in Pt-skin shell only by XRD. However, we have demonstrated the existence of Pt-skin as well as its lattice compression by HAADF-STEM images, Aberration-Corrected HRTEM, EDS mapping & line-scanning profile and CO stripping results (Please see our detailed responses to Comments 4).
In the revised manuscript, we removed the inaccurate description "After annealing in argon/hydrogen mixture at 300°C, the Pt diffraction peaks showed a slight negative shift (about 0.4°), further confirming the successfully generating compressive lattice strain in the Pt-skin shells." And, we provided new TEM data and detailed analysis for the Pt-skin structure (as well as its lattice compression) in Figure

Response:
We appreciate the reviewer for the kind suggestion. The bimetallic alloy characteristic/crystallinity nature of the Pt nanowire and other core-shell alloyed samples are confirmed by XRD. As shown in Figure   2e. the diffraction peaks of (111), (200), (220) and (311) are indexed to face centered cubic (fcc) structured Pt and PtNi alloy (Pt,. When compared to pure Pt nanowire, the XRD diffraction peaks of core-shell PtNi alloy CSFWS etc.) shift to higher angles with wider width (in consistence with the previous literature, e.g., Chem. Commun., 2021, 57, 623-626;Front. Energy, 2017, 11, 260-267), which could be attributed to the decrease of lattice distance when smaller Ni atoms alloyed with Pt atoms in the lattice and the resulting lower crystallinity.
Moreover, according to the Scherrer equation, the average crystalline domain size of Pt nanowires was estimated to be ~3.5 nm, which is consistent with the diameter obtained from the TEM micrograph in In the revised manuscript, we added a detailed explanation for the sharper XRD peak of Ptnanowire compared with other core-shell alloyed samples on Page 7-8.

Comment 3:
The authors failed to describe how the d-band center was obtained from the data presented in Figure 2f. The interpretation of the d-band center is also problematic because the energy of d-band center is referenced to the Fermi level, i.e., a negative number. If the 0 binding energy in Figure 2f -Band Center Via Transition Metal Oxide Support Interactions. ACS Catal. 2021, 11 (15), 9317-9332.?

Response:
We greatly appreciate the reviewer for pointing out this issue. The d-band center is calculated according to the following equation: = where E is the binding energy, R(E) is the UPS intensity after background subtraction, Ef is the Fermi energy level, and the calibration of the UPS revealed that the Ef of all samples was approximately 0.  Fig. 3h confirmed a well-defined Pt-skin formed on top of the ultrathin Pt3Ni curves framework shells. However, this image is problematic. The inset is a dark field image while Fig. 3h apparently, is a bright field image. In a bright field image, Z-contrast does not exist. So one cannot conclude that the red-doted region is Pt-rich. Furthermore, it is very hard to tell where exactly Fig h is located in the inset. They don't seem to be coming from the same area. So the repetitive claim of Pt skin@ Pt3Ni has little supporting evidence. It is suggested that the author tune down their emphasis on the skin structure or provide stronger evidence of the claimed Pt-skin, including those mentioned in the discussion of XRD.

Response:
We thank the reviewer for pointing out our poor demonstration of the Pt-skin structure formed on surface of the ultrathin Pt3Ni curves framework shells. To unambiguously identify the Pt skin, we employed high angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), in conjunction with energy-dispersive X-ray spectroscopy (EDX), to achieve elemental mapping & line-scanning over a single mesoporous Pt@Pt-skin Pt3Ni CSFWs. Moreover, the successful formation of Pt-skin structure was also identified by electrooxidation of carbon monoxide (CO stripping) and underpotentially deposited hydrogen (HUPD). On a Pt-skin surface, H UPD exhibits lower surface coverage due to weakened binding, which yields a characteristic ECSA CO :ECSA HUPD ratio of approximately 1.5 (J. Am. Chem. Soc. 2015, 137, 15817−15824, Science,2014, 343, 1339-1343. This has proven to be an easy way to identify formation of the Pt-skin surface structure over Pt3Ni. As shown in Figure 4c and Supplementary Table 2  Comment 5: Please execute reproducibility studies on the RDE testing. Error bars should be provided in the bar chart (Fig 4d).

Response:
We appreciate the reviewer for the kind suggestions. As suggested, all catalysts were evaluated for reproducibility of ORR activity in 0.1 M HClO4 electrolyte by using the optimized ink formulation. Table 3 show the reproducibility of the CV curves, ORR polarization curves, specific and mass activity for 4 samples (Pt/C, Pt NWs, mesoporous Pt@Pt3Ni CSFWs and mesoporous Pt@Pt-skin Pt3Ni CSFWs), and each sample is measured for 5 independent thin-film electrodes in 0.1 M HClO4 electrolyte. The average mass activity for Pt/C, Pt NWs, mesoporous Pt@Pt3Ni

Supplementary Figure 22-24 and Supplementary
CSFWs and mesoporous Pt@Pt-skin Pt3Ni CSFWs was 0.23 ± 0.012, 1.2 ± 0.045, 2.18 ± 0.062 and 6.69 ± 0.083 A mg -1 Pt, respectively. The average specific activity for Pt/C, Pt NWs, mesoporous Pt@Pt3Ni CSFWs and mesoporous Pt@Pt-skin Pt3Ni CSFWs was 0.33 ± 0.016, 1.92 ± 0.063, 3.2 ± 0.09 and 8.42 ± 0.126 mA cm -2 , respectively. The above RDE testing clearly confirms that the ORR activity for the same catalyst is reproducible and comparable to each other. Comment 6: The performance of Pt/C and Pt nanowires risk underestimation since the poor maintenance of limiting current typically reflects a poorly deposited catalyst film on the RDE electrode. This causes a mass transfer problem and resulted in the early experiencing of diffusion resistance (mixed diffusion/kinetic region) in the catalyst layer. The authors should re-do the measurements for Pt/C and Pt nanowires to provide a reasonable control group.

Response:
We appreciate the reviewer for pointing out this issue. Motivated by the reviewer's suggestions, we redo the ORR measurements for both commercial Pt/C (20 weight % Pt on Vulcan XC-72 carbon support, Pt particle size 2 to 5 nm) and Pt nanowires. For the sake of accurate comparison, all the uniform catalyst thin-films over the whole electrodes are prepared by using exactly the same rotational drying method (please see the experiment section for details). This rotational drying method is a simple and reliable for depositing thin layers of electrocatalysts for excellent performance in RDE methodology. Typically, the welldispersed Pt/C based ink was dried with a rotation rate of 700 rpm for at least 20 minutes (at room temperature in air) to deposit high-quality Pt/C thin film on glassy carbon disk electrodes. Based on five independently tested commercial Pt/C electrodes, the average mass activity and specific activity for Pt/C was measured to be 0.23 ± 0.012 A mg -1 Pt and 0.33 ± 0.016 mA cm -2 , respectively, which is well Comment 7: The author should explain how the mass loading of Pt on the RDE is determined since the errors would be amplified twice. A higher loading would give a much better kinetic current in the ORR measurement. Therefore, if the loading was underestimated, the mass activity would be overestimated significantly. The relation between catalyst loading and measured mass activity is typically non-linear. To obtain a fair comparison, the electrode loading should be the same (difference within 10%).
In addition, based on the provided CV curves in Figure 4a, the Pt@Pt-skin Pt3Ni CSFWs showed a double layer much larger than other samples. Although it is possible that the porous framework may exhibit more surfaces compared with other types of Pt nanostructures, carbon black should still dominate the double layer region due to the rather low loading of Pt (20wt%). The reviewer highly suspects that there are substantial errors in the determination of the electrode loading. It is recommended, if accessible, that the authors utilized instruments like XRF to directly measure the loading of Pt on the catalyst-coated RDE.

Response:
We appreciate the reviewer for pointing out this issue. In our work, the mass loading of Pt on the RDE was determined by the inductively coupled plasma-optical emission spectrometer (ICP-OES) measurement, which is widely recognized as an effective measurement to quantify the metal loading in the Pt supported electrocatalysts (Nature Mater. 2016, 15, 1188-1194; Science, 2019, 366, 850-856.   Nature 2021 598, 76-81. etc.). Motivated by the reviewer's suggestions, we re-do the ORR measurements for all four samples(Pt/C, Pt NWs, mesoporous Pt@Pt3Ni CSFWs and mesoporous Pt@Pt-skinPt3Ni CSFWs), and the Pt loading for all samples was strictly controlled to be 6.5 μg/cm 2 based on ICP-OES measurement. As shown in Figure 4a in the revised manuscript, the new CV curves of the four samples based on the same Pt loading exhibit almost the same CV double layer, which guarantees a fair comparison of the ORR performance for the four samples. In addition, we retested each catalyst for 5 independent thinfilm electrodes to ensure the reproducibility of performance data.

Response:
We appreciate the reviewer for pointing out this issue. In the previous manuscript, we fitted and extrapolated the Tafel curves to enable the Tafel plots of all catalysts in the same potential region from 0.8 -0.96 V, so that the Tafel slopes of all samples seem similar. Motivated by the reviewer's suggestion, we retested the ORR performance of all catalysts and recalculated their Tafel slope near half-wave potential region in LSV. As shown in Figure 4b in the revised manuscript, the Tafel plots of specific activity exhibit slopes of 51.9, 63.4, 69.2 and 80.9 mV dec -1 for Pt@Pt-skin Pt3Ni CSFWs, Pt@Pt3Ni CSFWs, Pt NWs and Pt/C electrocatalyst, respectively. A considerably smaller Tafel slope achieved in the Pt@Pt-skin Pt3Ni CSFWs suggests significantly improved kinetic for ORR. Comment 9: How come there the Pt oxide reduction peak is more pronounced during the CO stripping experiment? CO-stripping results should also be provided for other samples.

Response:
We appreciate the reviewer for pointing out this issue. In our previous manuscript, the reduction peak of Pt oxide was found more pronounced in the CO-stripping curve than that in CV curve, which may probably be ascribed to the following 3 factors: (1) The degree of activation of the catalysts. The activation degree of the Pt@Pt-skin Pt3Ni CSFWs catalyst prior to CO-stripping testing may probably be a little bit more sufficient than its CV activation in N2-saturated electrolyte. Therefore, compared with CV test, more Pt active sites exposed on the Pt@Pt-skin Pt3Ni CSFWs for CO stripping will result in its more pronounced Pt oxide reduction peak.
(2) The degree of N 2 saturation of HClO 4 solution. The CO-stripping and CV are both tested in N2saturated 0.1M HClO4 solution. For CO-striping experiment, the degree of N2 saturation of HClO4 solution may probably be lower than CV, which enables more Pt oxide formation during the positive-direction scanning. Correspondingly, the Pt oxide reduction peak is more pronounced during the CO stripping under negative-direction scanning.
(3) The sealing of the test system. There will be slight changes for sealing of the CO-stripping and CV test system from time to time, which may induce more dissolution of contaminated O2 in the reaction during the CO-stripping test, finally resulting in the more pronounced Pt oxide reduction peak during the CO stripping under negative-direction scanning.

Supplementary Table 2. Comparison of ECSACO and ECSAH UPD among the catalysts.
Comment 10: The accelerated durability cycles protocol is composed of 50,000 CV cycles from 0.6 to 1.1 V vs RHE. The reviewer is quite surprised that there is almost no sign of carbon corrosion based on the CV provided before and after the ADT. Some degree of carbon corrosion was observed for the Pt@Pt3Ni CSFWs/C while severe carbon corrosion occurred on the Pt/C. It would be nice if the authors could explain more on this part. The durability testing results for the Pt nanowires should be provided.

Response:
We thank the reviewer very much for this useful comment. Compared to commercial Pt/C and Pt@Pt3Ni CSFWs/C, we think that the negligible sign of carbon corrosion of Pt@Pt-skin Pt3Ni CSFWs may originate from the following factors:

Response:
We thank the reviewer for raising this comment. In the present work, the accelerated durability testing (ADT) was performed in an O2-saturated 0.1 M HClO4 solution at room temperature. After ADT, we wipe samples from the surface of the working electrode with cotton soaked in ethanol and collect them in a glass vial. The catalysts were then re-dispersed in ethanol by ultrasonication and collected finally by centrifugation.
Yes, all catalyst samples after the ADT were processed using the same way described above.

Response:
We appreciate the reviewer for the comments. In the revised manuscript, we have carefully addressed all the reviewers' comments and accordingly revised our manuscript. We believe the revised manuscript reserves the high quality and can meet the high standards in Nature Communications.

Response:
We thank the reviewer for raising this comment. The solvothermal method for synthesis of shapecontrolled nanoparticles has aroused intense interest and admittedly widely reported in recent years, whereas our Mesoporous Core-shell Framework Nanowires (CSFWs) configuration has not yet been reported before. We designed a unique mesoporous Pt@Pt-skin Pt3Ni CSFWs with integration of 3D open mesoporous configuration, 1D anisotropic core-shell motif and lattice strained Pt-skin surface. This rational design of CSFWs nanostructure is unprecedented and endows ultrahigh electrochemical active surface area and activity. Especially, the CSFWs configuration exhibits superior electrocatalytic stability with a negligible activity decay (less than 3%) after 50,000 cycles, which has proven to be one of the bestknown ORR electrocatalysts to date.

Comment 2:
The authors must provide a much more detailed description of the various effects of time/temperature/molar ratios of capping/reducing agents onto the shape of the final catalyst, as for example shown by Yong Yang et al. (J. Phys. Chem. C 2007, 111, 26, 9095-9104), combined with the various achieved structures and possibly a volcano plot of the corresponding ORR activities. Without this essential information, it is very difficult to understand the various factors that lead to the shape and activity of the final catalyst.

Response:
We greatly appreciate the reviewer for the useful suggestion. As suggested by the reviewer, we cited the "Yong Yang et al., J. Phys. Chem. C 2007, 111, 26, 9095-9104" paper as an important reference (ref.34) in our revised manuscript. Motivated by the reviewer's suggestion, various influence factors (such as time/temperature/molar ratios of capping/reducing agents) onto the morphology of the Pt-Ni nanowires catalysts were studied systematically in our revised manuscript. Figure 11, the time-dependent morphology changes results revealed that the formation of the well-defined Pt@Pt-Ni CSNWs experienced the initial formation of ultrathin Pt nanowires (0.5 h), the deposition of Ni-rich phase onto the performed Pt nanowires (2 h-12 h), and finally complete reduction of Pt/Ni precursors to form nanogourd-string-like Pt@Pt-Ni alloy CSNWs (24 h). Figure 12, the temperature-dependent morphology changes results revealed that the length of the nanogourd-string-like Pt@Pt-Ni alloy CSNWs increased from tens of nanometers (160°C) to hundreds of nanometers (180°C), and finally grow to a few microns (200°C).

As shown in Supplementary
However, when reaction temperature further increased to 220°C, an uneven nanogourd-string-like Pt@Pt-Ni alloy CSNWs and nanoparticles mixture will be formed.
The use of CTAB also plays a critical role in determining the morphology of the Pt-Ni nanowires. As shown in Supplementary Figure 13, without the addition of CTAB, only irregular polyhedral nanocrystals (NCs) with an average size of 30 ± 5 nm were produced. After the addition of 10 mg CTAB, some nanogourd-string-like Pt@Pt-Ni CSNWs mixed with NCs were observed. The uniform nanogourd-stringlike Pt@Pt-Ni CSNWs (with an average diameters of 24.3 nm) in a very high yield can be obtained with 40 mg CTAB. While further increasing the CTAB amount to 50 mg, the Pt@Pt-Ni CSNWs yield decreased and the morphology becomes irregular. Therefore, it can be concluded that CTAB was the structure-directing agent and a certain amount of CTAB controlled the growth of nanogourd-string-like Pt@Pt-Ni CSNWs.
Glucose as a reducing agent is also critical to the final morphology of Pt@Pt-Ni alloy core-shell nanowires (CSNWs). As shown in Supplementary Figure 14 Supplementary Fig. 25 showed the volcano-shaped ORR activity relationship, and the uniform mesoporous Pt@Pt-skin Pt 3 Ni CSFWs/C obtained by adding 8 mg Ni(acac) 2 achieves the best ORR performance.
In the revised manuscript, we added TEM images of time/temperature/molar ratios of cappingdependent morphology changes in Supplementary Figure 11-15 Comment 3: While the reported ORR activity appears impressive compared to 20wt% Pt/C, a comparison to other -highly active shape controlled -Pt3Ni/C catalyst, for example in a tabulated form, is missing, which would allow direct ORR activity comparison with published results. As for example, Pt sub-nanometer Pt alloy wires with 4.20 A/mg & 5.11 mA/cm 2 at 0.9 V vs RHE and very high durability over 30000 cycles were already reported in 2017 (Science Advances, https://doi.org/10.1126/sciadv.1601705).

Response:
We gratefully appreciate the reviewer for this valuable comment. Motivated by the reviewer's suggestion, the comparison of the ORR activity of mesoporous Pt@Pt-skin Pt3Ni CSFWs/C with other Pt3Ni/C catalysts published in recent years was provided in Supplementary Table 4. Considering comparable Pt loadings, the performance of mesoporous Pt@Pt-skin Pt3Ni CSFWs/C is among the best reported performance for Pt3Ni/C catalysts. Table 4   : The durability loss of the catalysts is obtained after 8000, 50000, 4000, 20000 and 6000 potential cycles, respectively.

Response:
We thank the reviewer very much for the useful suggestion. Motivated by the reviewer's suggestion, the size distribution (with error bar) of initially formed Pt nanowire, Pt@Pt-Ni alloy CSNWs and final mesoporous Pt@Pt-skin Pt3Ni CSFWs was provided in Supplementary Figure 10 in the revised Supporting Information. As shown in Figure R2 and Supplementary Figure 10

Comment 5: Specifically, I would like to add the following questions/comments to the authors:
Lines 77/78: Standard reduction potentials are given for specific conditions (pH) and aqueous solutions of the metals, therefore under the given reaction conditions they vary. Secondly, what exactly triggers the onset of Ni-reduction after the initial Pt-reduction, only the depletion of Pt cations?

Response:
We appreciate the reviewer very much for the commments.
On the first question, the reduction potential admittedly varies with reaction conditions. However, for the statement "... Since a more positive reduction potential of the Pt 2+ /Pt (1.18 eV versus RHE) relative to Ni 2+ /Ni (-0.257 eV versus RHE) ...", we want to emphasize that the reduction potential of Pt ion is positive to Ni ion under the same reaction conditions in our work, inducing a preferential reduction of Pt ions relative to Ni ions.
On the second question, there are two major factors motivating the onset of Ni-reduction after the initial Pt-reduction. (1) The largely depletion of the Pt precursor accelerates the reduction of Ni ions. (2) Previously reduced Pt serves as a catalyst for Ni reduction. Peng et al. studied the growth path of octahedral Pt3Ni nanoparticles by using in-situ ambient pressure X-ray photoelectron spectroscopy (AP-XPS) and X-ray absorption spectroscopy (XAS), confirming that previously reduced Pt has a catalytic effect on the reduction of Ni (Nat. Commun. 2018, 9, 4485).
For the purpose of further demonstration, we also compared the reduction ability of Pt(acac)2/Ni(acac)2 mixture, pure Pt(acac)2 and pure Ni(acac)2 under the same reaction conditions, respectively. This method has also been reported by Gong, M. et al. before (ACS Catal. 2019, 9, 4488-4494.). As shown in Supplementary Figure 1, the reduction of Pt(acac)2/Ni(acac)2 mixture and pure Pt(acac)2 begins at approximately 160°C. However, the pure Ni(acac)2 cannot be reduced even under high reaction temperature up to 200°C, profoundly demonstrating that Pt 2+ ions have more positive reduction potential than that of Ni 2+ ions under the same reaction conditions in our work, and the previously reduced Pt crystal nuclei serves as a catalyst for Ni 2+ ions reduction.

Response:
We thank the reviewer for raising this comment. Since Pt and Ni belong to the same main element family, signal interference of Ni is inevitable in EDS line scanning. The Ni signals appearing in the EDS line-scanning profile in Supplementary Figure 4 are actually the background noise. As suggested, in order to profoundly demonstrate the absence of Ni phase in the initially formed Pt nanowires, the corresponding EDS mapping and intensity images were provided (Supplementary Figure 4). As shown in Supplementary   Figure 4, Pt is the most dominant element in the nanowires, while Ni only exists as background noise. In addition, the EDS intensity images illustrate that all Ni element peaks (Ni-Kα, Ni-Kβ, Ni-Lβ peaks) are background noise. Consequently, we consider that there is no Ni element in the initially formed nanowires.

Response:
We thank the reviewer for raising this comment. Yes, we fully agree with the review that the XRD peaks splitting can be interpreted as phase separation or possibly some de-alloying in the synthesis process.
However, de-alloying is commonly regarded as the preferential dissolution (or removal) of the electrochemically more active component from a bimetallic alloy (Nature Chem.2010, 2, 454-460;Science 1991, 254, 687-689;Nature 2001, 410, 450-453;Nature 2006, 430, 707-710). Whereas, the phase separation is referred to as a multi-component system sometimes separate into several phases with different components and structures when external conditions such as temperature and pressure change. In the present work, the Pt@Pt-Ni alloy CSNWs were obtained by hydrothermal condition, without any selective etching treatment in a corrosive medium. Therefore, the observed XRD splitting of Pt@Pt-Ni CSNWs in Figure 2e is probably caused by phase separation (Pt-rich phase and Ni-rich phase).
Comment 8: Line 129: This is confusing because the final synthesis product, Pt@Pt-skin Pt3Ni CSFWs are obtained after the annealing step, to obtain the Pt-skin structure. If a slight negative shift of 0.4º was observed for the final catalyst, why are the data not presented in figure 2 e?

Response:
We thank the reviewer for raising this comment. As response to the comment 1 raised by Review 1#, the Pt-skin shell could not be precisely determined by XRD. In the present work, the Pt-skin shell only affects a few atomic layers on the surface of the mesoporous Pt@Pt-skin Pt3Ni CSFWs sample, resulting in its absence of significant peak shift of the XRD pattern when compared to Pt@Pt3Ni CSFWs (Supplementary

Response:
We thank the reviewer for raising this comment. As response to comment 4 raised by Review 1#, we let the tip of the salt bridge as close as possible to the working electrode to reduce the ohmic voltage drop (distance should not be less than the outer diameter of the salt bridge, otherwise it will have a shielding effect on the surface of the working electrode). Then we carry out IR compensation to reduce the effect of solution resistance (IR compensation was generally maintained within 90% to avoid self-excitation of the electrochemical workstation).
(2) Solution environment: LSV curves were tested in N2 and O2 saturated electrolyte, respectively. To eliminate the effect of background current, the final LSV curves are obtained by subtracting the N2-saturated data from the O2-saturated data.
(3) Catalyst preparation: The catalysts and carbon black should be well mixed by ultrasonic treatment.
In the represent work, all the uniform catalyst thin-films over the whole electrodes are prepared by using the same rotational drying method (please see the experiment section for details).
(4) Other factors: We utilize Ag/Cl and graphite as reference electrode and counter electrode (anode) respectively and keep the test environment at room temperature as well as maintaining a good sealing of the test system, etc.
In 2012, Michael C. Carpenter et al., reported the solvothermal synthesis of Pt alloy nanoparticles with the focus on Pt-Ni alloys by using DMF as solvent and reductant. They obtained well-faceted predominantly cubic and cuboctahedral nanocrystals of PtNi alloy with high ORR activity. (Michael C. Carpenter et al., J. Am. Chem. Soc. 2012, 134, 8535−8542, dx.doi.org/10.1021/ja300756y) and showed how in variation of reaction parameters, different structures were obtained. Therefore, in their introduction, the authors should add some more sentences to make more clear and discuss by citing additional literature the difference to published results and the striking novelty in their approach and obtained catalyst structure, respectively.
Response to other comments #2: The authors have addressed the question very well and now the effect of the various parameters become clear as also their effect on the final catalyst structure and electrochemical ORR activity. About SI figure 20, the K-L plot refers to SI 20a or 19a? #3: The issue has been addressed #4: The issue has been addressed #5: The issue has been addressed #6: By the additional data provided by the authors it has become clear now that Ni is absent in the initial phase of the reduction process #7: The issue has been addressed #8: The issue has been addressed #9: The issue has been addressed #10: The way in which a variation of the Ni content effects the formed structures has become clear now.
#11: The issue has been addressed #12: The issue has been addressed #13: The issue has been addressed #14: The electrode loading has been reported now #15: Additional information has been provided and it became clear now.

Point-by-point responses to the Reviewers' Comments
Reviewer 1#

General Comments
The authors have gone through the reviewer's comments in detail. The two major concerns pointed out by the reviewer have been addressed thoroughly and adequately. The high-resolution HAADF-STEM characterization of the materials indeed provided much stronger evidence for the claimed Pt-skin structure. The RDE testing was also conducted much more rigorously with results of much higher quality. The reviewer thinks the revised manuscript is of high quality and deserves to be published at Nature Communications.

Response:
We appreciate the reviewer for the positive comments and his kind recommendation for publication of our revised manuscript without any further revision in Nature Communications.

General Comments
In the revised manuscript "Mesoporous Pt@Pt-skin Pt3Ni Core-shell Framework Nanowires for High-Efficient Electrocatalysis" by Hui Jin et al. the authors present a largely revised version of their original manuscript. However, regarding my comment #1 in my first review "Other than the achieved shape of the catalyst, I cannot find new aspects in the research that would justify publication in Nature Communications, the synthesis method (solvothermal) has been reported already for synthesis of shapecontrolled NP." I still do not clearly see the novelty that would justify publication in Nature Communications. In their response to my comment, the authors wrote "The solvothermal method for synthesis of shape-controlled nanoparticles has aroused intense interest and admittedly widely reported in recent years, whereas our Mesoporous Core-shell Framework Nanowires (CSFWs) configuration has not yet been reported before." Is the novelty the synthesis method of applying the solvothermal method in combination with a special capping reagent to obtain specifically core-shell nanowires consisting of two different metals or is it the obtained anisotropic mesoporous structure itself? (ACS Appl. Mater. Interfaces 2018, 10, 40, 34147-34152, https://doi.org/10.1021 about "Mesoporous Pd@Ru Core-Shell Nanorods for Hydrogen Evolution Reaction in Alkaline Solution", in which they describe a synthesis route where the mesoporous nanorod structure was obtained starting from Pd nanorods as seeds to synthesize the Pd@Ru core-shell nanorods by a diffusion process during a 12 h reaction period in a Teflon-lined stainless-steel autoclave under 200 ºC. While the authors do not explicitly use the term "solvothermal", the procedure they use is, according to the definition "Solvothermal synthesis is defined as a chemical reaction that takes place in a solvent at a temperature higher than the boiling temperature of the solvent in a sealed vessel." (Cited from "Chemical Solution Synthesis for Materials Design and Thin Film Device Applications", 2021, Pages 79-117, Chapter 3 -Deposition of thin films by chemical solution-assisted techniques, Ankit Kashyap et al., https://doi.org/10.1016/B978-0-12-819718-9.00014-5) a solvothermal procedure. In 2012, Michael C. Carpenter et al., reported the solvothermal synthesis of Pt alloy nanoparticles with the focus on Pt-Ni alloys by using DMF as solvent and reductant. They obtained well-faceted predominantly cubic and cuboctahedral nanocrystals of PtNi alloy with high ORR activity. (Michael C. Carpenter et al., J. Am. Chem. Soc. 2012, 134, 8535−8542, dx.doi.org/10.1021 and showed how in variation of reaction parameters, different structures were obtained.

Response:
We appreciate the reviewer for the comments. The novelty of our work mainly focuses on the well-defined anisotropic mesoporous Pt@Pt-skin Pt3Ni core-shell framework nanowire structures (CSFWs), which has not yet been reported before.
Such unique CSFWs configuration combines the advantages of 1D ultrathin atomicjagged Pt nanowire (diameter ~ 3 nm) core and 3D open lattice-strained Pt-skin (~1-1.5 nm) Pt3Ni framework shell, which exhibits distinctive mass activity (6.69 A/mgpt) and specific activity (8.42 mA/cm 2 ) toward ORR, nearly 29 and 26 times higher as compared with the state-of-the-art commercial Pt/C catalyst. The catalyst also exhibits high stability with negligible activity decay after 50,000 cycles, which has proven to be one of the best-known oxygen reduction reaction (ORR) electrocatalysts to date.
As mentioned by reviewer, in 2018, Luo et al. also reported a 1D mesoporous Pd@Ru core-shell nanorods, which exhibit the competitive hydrogen evolution reaction (HER) catalytic activity and stability. However, the morphology, structure